1. Field of the Invention
The present invention relates to a cylinder head for a DOHC engine, and more specifically to a cylinder head structure for providing a higher camshaft supporting rigidity.
2. Description of the Prior Art
To obtain higher engine performance in automotive vehicles, DOHC (double overhead camshaft) engines have been widely used, in which two different camshafts for driving intake and exhaust valves separately are provided on a cylinder head. An example of these DOHC engines is disclosed in Japanese Published Unexamined (Kokai) Utility Model Application No. 61-17162, for instance. In this disclosed DOHC engine, as shown in FIG. 1 two camshafts 2A and 2B are rotatably supported on two bearing portions 3A and 3B formed on a cylinder head 1, and a plurality of ignition plugs (not shown) are inserted into plural cylindrical plug insertion bosses 4 formed along the central line of the cylinder head 1. Further, a number of reinforcement ribs 5A and 5B are formed integral with the cylinder head 1 so as to extend between the bearing portions 3A and 3B and the cylindrical plug insertion bosses 4 in order to increase the rigidity of the cylinder head 1 at which the two camshafts 2A and 2B are supported.
In the prior-art cylinder head as described above, however, since no cylindrical plug insertion bosses 4 can be formed at the bearing portions on both the front and rear sides along the cylinder arrangement line of the cylinder head 1, it is impossible to form the above-mentioned reinforcement ribs 5 on both the sides of the cylinder head 1. In the cylinder head for a DOHC engine, however, since camshaft driving elements such as chain wheels, chains, belt pulleys, etc. are usually attached to the front end of the camshafts 2A and 2B in order to drive the camshafts, a torque is inevitably applied to each camshaft by the camshaft driving elements therefore a load is applied in the obliquely downward direction at the camshaft bearing portions and therefore the front end camshaft bearing portions are deformed, so that the camshafts are scratched by the deformed camshaft bearing portions of the cylinder head. In other words, in the prior-art cylinder head for a DOHC engine, there exists a problem in that the rigidity of the cylinder head 1 is not sufficient at the camshaft bearing portions 3A and 3B and therefore the camshaft supporting surfaces are deformed or the camshafts are damaged due to journal scuff (scratches).